Sound reproduction devices which operate in accordance with the flexural wave principle are known in the state of the art. Such devices are essentially composed of a panel and at least one drive system, and the panel oscillates when sound signals are transmitted to the drive system or systems. It is characteristic for such sound reproduction devices that a flexural wave radiation starts at a critical lower frequency, where the flexural wave leads to the radiation of sound in a frequency-dependent direction along the plane of the respective panel. In other words, a cut through a directivity diagram shows a principal lobe whose direction is frequency-dependent. These relationships apply fully to infinitely expanded plates and absorber plates, while the relationships for the multi-resonance plates covered by this application are clearly more complex because of the strong edge reflexes. This complexity of the multi-resonance plates is due to the fact that the named principal lobe has a number of such further principal lobes superimposed on it in a frequency-dependent direction, so that a highly fan-shaped directivity diagram is created which furthermore is very frequency-dependent. However the multi-resonance plates and the absorber plates treated here have in common that the center of their directivity diagram rather points away from the mid-perpendicular. This property causes the room to have a stronger effect on the projection of the sound waves.
The panel is constructed according to the sandwich principle, where each of two opposite surfaces of a very light core layer are attached to a thin cover layer, for example by means of an adhesive. In order for the panel to have good sound reproduction properties, the material for the cover layer must have an especially high dilatational wave speed. Suitable cover layer materials are for example thin metal foils or fiber-reinforced plastic foils as well.
Special demands are also made on the core layer. Thus in the first place it is necessary for the materials being used to have a low mass density and low damping. In addition the materials for the core layer must have as high a vertical shear modulus as possible with respect to the sides which are provided with the cover layers. Finally in the sense of a principal requirement, it is necessary for the materials that can be used for the core layers to have a very low modulus of elasticity along the direction in which the greatest expansion of the subsequently formed core layer takes place. These two premises, which at first glance are contradictory in reference to the last two requirements, are better fulfilled by a core layer which has a perforated structure with openings of a preferably small cross section which extend between the two surfaces to be covered by the cover layers. In addition to the core layers with the perforated structure, hard foams can also be used as core layer materials because despite their isotropic properties, these materials still exhibit suitable shear and elasticity moduli. In this connection it should also be mentioned that when hard foams are used as the core layer material, the objective of the cover layers is to produce the necessary anisotropic behavior of the panel.
It was the basic idea of the sound reproduction according to the flexural wave principle to use existing walls to radiate the sound. It was first assumed that to produce flexural waves it would suffice to equip the corresponding walls with drive systems. It was soon learned that only walls which had been optimized for this purpose were in a position to guarantee a satisfactory sound radiation. But a prerequisite was that walls optimized in this manner would be available in sufficient size to prevent damping the propagating waves against a rigidly boxed panel. However it is often the case that surfaces which are able to receive a panel of the above described size are not available. To enable the use of small panels as well for an acceptable sound reproduction, it is necessary to attach these to a holder by means of flexible fasteners. To that end the beads known from loudspeaker technology, which connect the edge of the panel with the holder, could be used. Aside from the fact that the known beads represent an additional component, these beads are also a disadvantage because the steps needed to install the beads make the production process considerably more expensive. Irrespective thereof, the edge attachment of smaller size panels is also disadvantageous if the corresponding panel is equipped with a number of drive systems, as is the usual practice. This measure increases the weight of the already heavy panel, and the beads must therefore be very hard to sufficiently suspend the panel, which at the same time increases the damping that is caused by these beads.